1. Field of the Invention
The present invention is related to the improvement of the oxidation stability of polyol ester based, lithium soap thickened greases by use of combination antioxidant and to the greases containing such additives.
2. Description of the Related Art
The continued improvement in high efficiency equipment has placed a severe operating demand on the lubricants used in such equipment. High efficiency is, in most instances, achieved by designing the equipment to run at high speeds, higher temperatures, longer times between servicing, leaner fuel settings, etc., all of which increase the demands placed on the lubricants used. The severe performance demands placed on the lubricating greases operating at higher temperatures for longer times can be met primarily by improving the grease oxidation stability.
U.S. Pat. No. 4,298,481 discloses a high temperature grease composition comprising a base fluid that is a C.sub.5-16 dialkyl ester of hydrogenated dimer acids that contain less than 8% by weight of trimer acids, an additive system and a thickener comprising an oleophilic surface-modified clay. The additive system consists of antioxidant, rust-inhibiting, metal passivating and load bearing components. The additive system is used in an amount in the range of 0.2 to 6 wt %. The system contains 0.1 to 2% of an antioxidant, said antioxidant being one or more aromatic amine antioxidant(s) alone or in combination with a hindered phenol, organic phosphate, alkyl thiodialkanoate and/or other conventional antioxidants. Suitable aromatic amine antioxidants include phenothiazine and substituted phenothiazines, diphenylamine, dinaphthylamine, p,p'-dioctyldiphenyl amine, etc. and mixtures thereof.
U.S. Pat. No. 3,663,438 discloses a high temperature grease composition comprising a mineral oil base stock thickened to grease consistency and incorporating minor amounts of a phenothiazine type oxidation inhibitor and a polyester of C.sub.1 -C.sub.30 alcohol and C.sub.3 -C.sub.20 carboxylic acids. The thickener used can include a wide variety of commonly accepted materials including soap-based thickeners, organic thickeners and clay thickeners.
U.S. Pat. No. 5,319,081 discloses substituted N-thiomethylphenothiazines as lubricant anti-oxidation stabilizers. The substituted N-thiomethylphenothiazine may be used in combination with other known antioxidant additives such as aromatic amines (e.g., p-tert octylphenyl-.alpha.-naphthylamines, p,p'-ditert octyl diphenylamines, 2,3-dihydro-3,3-dimethyl-4H-1,4 benzothiazine, phenothiazine, diphenylamine, etc.), hindered phenols, aliphatic or aromatic phosphates, esters of thiodipropionic or thiodiacetic acid, or salts of dithiocarbonic or dithiophosphoric acids. The patent broadly states that such antioxidant combinations may show a synergistic action and that such performance can be obtained when combining the substituted N-thiomethyl-phenothiazine of the patent with certain aromatic amines or hindered phenols or both. No direction is given as to how to select synergistic combinations nor are examples of synergistic combinations presented in the patent text.
GB 1,420,824 discloses functional fluids suitable for use as gas turbine lubricants and the base stocks for producing such fluids. The patent discloses the use of a particular synthetic ester base stock comprised of a mixture of a dialkyl ester of isophthalic acid and a synthetic ester selected from the diesters of aliphatic dicarboxylic acids and monohydric alcohols and the esters of monocarboxylic acid and polyhydric alcohols. The patent goes on to state that exceptional gas turbine lubricants can be formulated by incorporating into the ester base stock an antioxidant package comprising a phenothiazine or a substituted phenothiazine in combination with a diaryl amine.
GB 1,438,482 discloses lubricating oil additives impacting superior oxidation resistance to the resulting formulated oil. The additive is of the general formula R--X or R--Y--R where, in the case of the compound of formula R--X, R is a secondary amine residue containing two aromatic groups directly attached to nitrogen and X is an aliphatic hydrocarbon substituent containing 3 or 4 carbon atoms, which substituent is attached to a nitrogen atom of the group R and has an ethylenic or acetylenic linkage in the .beta.-position to the nitrogen atom, and, in the case of a compound of formula R--Y--R, each R is independently as previously defined, and Y is an aliphatic hydrocarbon substituent containing 4 carbon atoms, which substituent is attached to a nitrogen atom of each group R and has an ethylenic or acetylenic linkage at the .beta.-position to each nitrogen atom. A substituted phenothiazine of formula ##STR1## is described where X is an allyl or propangyl group and each R.sub.1 is the same or different and is a hydrogen atom, a C.sub.4 -C.sub.12 alkyl group or a C.sub.4 -C.sub.12 alkoxy group provided that both R.sub.1 groups are not hydrogen.